Some areas restrict these calls (for instance, if I'm in China, I cannot receive Facebook calls but when I'm back in the States, I can). It used to be limited to some accounts, but I think now it's a feature that's rolled out to everyone. They do need to have the Messenger app installed in their phones, though. It is limited to Facebook users only. Just remember, these calls are 'free' but they use your data, so watch out if you have a data cap.